-SA node is self excitatory: this is due to high Na ions concentration in ECF and its membrane is leaky to Na ions . Because of this +ve Na ions enters itself and causes RMP to more than threshold leading to AP.

-AV node can discharge impulse at rate of 40-60/min and purkinje at 15-40 /min if they are not stimulated from outside source.

- The dicharge rate of SA node is faster than the self excitatory discharge rate of others. Each time SA node discharges, its impulses is conducted through AV node and purkinje fibers and also depolarises their membrane. The depolarisation occurs before their membrane can reach to self excitatory threshold thus SA node inhibits impulse generation from AV node and purkinje fiber. Thus SA node is known as the pacemaker of heart.

Abnormal/Ectopic pacemakers

-Pacemaker elsewhere than SA node is known as ectopic pacemakers

-When SA node is not functioning at that time impulse is generated from AV node

-If there is AV block impulse will be generated from purkinje at rate of 15-40/min but atria will be beating normally. Purkinje will self generate impluse only after 5-20 sec because previously its function was suppressed by SA node. So during this time ventricles will not function and there will be lack of blood to brain and person faints and after sometime regains consciousnes. This is known as Stokes Adams Syndrome